Water removal from gas mixtures



Jan. 5, 1965 R. M. MILTON 3,164,453

WATER REMOVAL FROM GAS MIXTURES Filed Dec. 18, 1959 4 Sheets-Shet 1 g .sE 0 m U E 2 g 3 1? g L: F 2 w 2 5 B 5 E g n o o 9 8 a h. 0 1

m 3g 8 \O I) w c llg 4 I: 5 8 3 Q D- u o 2 N w as a: s 2 r w (v ul z pwm v OOl/ 1 M s) oaaaosov 113mm .LHSIBM JNVENTOR.

ROBERT M. MILTON ATTORNEY 06 0 0 NO U6 n6 0 0 Mg 4 Sheets-Sheet 2 R. M.MILTON HATER REMOVAL mom GAS urxmzss Jan. 5, 1965 Filed Dec. 18, 1959INVENTOR. RQBERT M. MILTON BY MM ATTORNEY Jan. 5, 1965 R. M. MILTONWATER REMOVAL FROM GAS MIXTURES 4 Sheets-Sheet 3 Filed Dec. 18, 1959ZEOLITE A ABSORPTION CAPACITY For Various Temperature Ratiosin 'K)TEMPERATURE RATIO T T T qndT INVENTOR. ROBERT MILTON BY WM ATTORNEY.

Jan. 5, 1965 R. M. MILTON WATER REMOVAL FROM GAS MIXTURES 4 Sheets-Sheet4 Filed Dec. 18, 1959 ZEOLITE A ADSORPTION CAPACITY For VariousTempermure Ratios TEMPERATURE RATIO Tz/Tl lclndTz inK) FIG.4.

INVENTOR. ROBERT M; MILTGN ATTORNEY United States Patent 3,164,453 WATERREMGVAL FROM GAS MIXTURES Robert M. Milton, Bufialo, N.Y., assiguor toUnion Carbide Corporation, a corporation of New York Filed Dec. 18,195%, Ser. No. 860,599 3 Claims. (11. 55-35 This invention relates to amethod for adsorbing fluids and separating a mixture of fluids into itscomponent parts. More particularly, the invention relates to a method ofadsorbing water with adsorbents of the molecular sieve type. Still moreparticularly, the invention relates to a method for preferentiallyadsorbing water from a vapor mixture containing at least one memberselected from the group consisting of olefinic and normal saturatedaliphatic hydrocarbons containing less than four carbon atoms permolecule, air, nitrogen and hydrogen.

The present process is advantageous in drying vapormixtures, forexample, in drying natural gas so as to prevent hydrate formation inpipelines, thereby avoiding corosion problems. This process may also beused to dry cracked gas containing mainly light olefins to preventclogging of liquid-gas contact means in the low temperature column usedto recover ethylene.

Broadly, the invention comprises mixing molecules, in a fluid state, ofthe materials to be adsorbed or separated with at least partiallydehydrated crystalline synthetic metal-aluminum-silicates, which will bedescribed more particularly below, and effecting the adsorption of theadsorbate by the silicate. used in the process of the invention is insome respects similar to naturally occurring zeolites. Accordingly, theterm zeolite would appear to be appropriately applied to thesematerials. These are, however, significant differences between thesynthetic and natural silicates. To distinguish the synthetic materialused in the method of the invention from the natural zeolites and othersimilar synthetic silicates, the sodium-aluminum-silicate and itsderivatives taugh hereinafter to be useful in the process of theinvention will be designated by the term zeolite A. While the structureand preferred method of making zeolite A will be discussed in. somedetail in this application, additional information about the materialand its preparation may be found in an application filed December 24,1953, Serial No. 400,388, now US. Patent No. 2,882,243, having issuedApril 14, 1959, in the name of R. M. Milton.

It is the principal object of the present invention to provide a processfor the selective adsorption of molecules from fluids. A further objectof the invention is to provide a method whereby certain molecules may beadsorbed and separated by crystalline synthetic metalaluminum-silicatefrom fluid mixtures of those molecules and other molecules.

In the drawings:

FIG. 1 is a graph showing the amount of Water adsorbed versus thetemperature ratio T T for various cationic forms of zeolite A;

FIG. 2 is a graph showing the amount of adsorbed normal saturatedaliphatic hydrocarbons having less than four carbon atoms per molecule,versus the temperature ratio T /T for various cationic forms of zeoliteA;

FIGURE 3 is a graph showing the amount of C -C normal unsaturatedaliphatic hydrocarbons adsorbed versus the temperature ratio T /T forvarious cationic forms of zeolite A.

FIGURE 4 is a graph showing the amount of nitrogen adsorbed versusthetemperature ratio T T for various cationic'forms of zeolite A.

Certain adsorbents, including zeolite A, which selectively adsorbmolecules on the basis of the size and shape The synthetic silicate3,164,453 Patented Jan. 5, 1965 of the adsorbate molecule are referredto as molecular sieves. These sieves have a sorption areaavailable onthe inside of a large number of uniformly sized pores of moleculardimensions. With such an arrangement molecules of a certain size andshape enter the pores and are adsorbed while larger or differentlyshaped molecules are excluded. Not all adsorbent behave in the manner ofthe molecular sieves. Such common adsorbents as charcoal and silica gel,for example, do not exhibit molecular sieve action.

Zeolite A consists basically of a three-dimensional framework of $0.;and A10 tetrahedra. The tetrahedra are cross-linked by the sharing ofoxygen atoms so that the ratio of oxygen atoms to the total of thealuminum and silicon atoms is equal to two or O/ (Al-i-Si)-=2. Theelectrovalence of the tetrahedra containing aluminum is balanced bytheinclusion in the crystal of a cation, for example, an alkali oralkaline earth metal ion. This balance may be expressed by the formulaAl /(Ca, Sr, Ba, N32, K )=1 One cation may be exchanged for another byion exchange techniques which are described below. The spaces betweenthe tetrahedra are occupied 'by water molecules prior to dehydration.

Zeolite A may be activated by heating to effect the loss of the water ofhydration. The dehydration results in crystals interlaced with channelsof molecular dimensions .that offer very high surface areas for theadsorption of foreign molecules. These interstitial channels will notaccept molecules that have a maximum dimension of the minimum projectedcross-section in excess of about 5.5 A. Factors influencing occlusion bythe activated zeolite A crystals are the size and polarizing power ofthe interstitial cation, the polarizability and polarity of the occludedmolecules, the dimensions and shape of the sorbed molecule relative tothose of the channels, theduration and severity of dehydration anddesorption, and the presence of foreign molecules in the interstitialchannels.

Although there are a number of cations that may be present in zeolite Ait is preferred to formulate or synthesize the sodium. form of thecrystal since the reactants are readily available and water soluble. Thesodium in the sodium form of zeolite A may be easily exchanged for othercations as will be shown below. Essentially the preferred processcomprises heating a proper mixture in aqueous solution of the oxides, orof materials whose chemical compositions can be completely representedas mixtures of the oxides, Na O, A1 0 SiO and H 0, suitably at atemperature of about 100 C. for periods of time ranging from 15 minutesto hours or longer. The product which crystallizes from the hot-mixtureis filtered off and washed with distilled water until the effluent washwater in equilibrium .with the zeolite has a pH of from about 9 to 12.The material, after activation, is ready for use as a molecular sieve.

Zeolite A may be distinguished from other zeolites and silicates on thebasis of its X-ray powder diffraction pattern. The X-ray patterns forseveral of the ion exchanged forms of zeolite A are described below.Other characteristics that are useful in identifying zeolite A are itscomposition and density.

The basic formula for all crystalline zeolites where M? represents ametal and n its valence may be represented as follows: 1

r M: 2 O :AlzOsiXSiOgZYHzO In general a particular crystalline zeolitewill have values for X and Y that fall in a definite range. The value Xfor a particular zeolite will vary somewhat since the aluminum atoms andthe silicon atoms both occupy essen- Na O/Al O has varied as much as23%.

tially equivalent positions in the lattice. Minor variations in therelative numbers of these atoms do not significaritly alter the crystalstructure or physical properties of the zeolite. For zeolite A, numerousanalyses have shown that an average value for X is about 1.85. The Xvalue falls within the range l.85i0.5.

The value of Y likewise is not necessarily an invariant for all samplesof zeolite A particularly among the various ion exchanged forms ofzeolite A. This is true because various exchangeable ions are ofdifferent size, and, since there is no major change in the crystallattice dimensions upon ion exchange, more or less space should be available in the pores of the zeolite A to accomodate water molecules. Forinstance, sodium zeolite A was partially exchanged with magnesium, andlithium, and the pore volume of these forms, in the activated condition,measured with the following results:

The average value for Y thus determined for the fully hydrated sodiumzeolite A Was 5.1; and in varying conditions of hydration, the value ofY can vary from 5.1 to essentially zero. The maximum value of Y has beenfound in 75% exchanged magnesium zeolite A, the fully hydrated form ofwhich has a Y value of 5.8. In general an increase in the degree of ionexchange of the magnesium form of zeolite A results in an increase inthe Y value. Larger values, up to 6, may be obtained with more fully ionexchanged materials.

In zeolite A synthesized according to the preferred procedure, the ratioNa O/Al O should equal one. But if all of the excess :alkali present inthe mother liquor is not washed out of the precipitated product,analysis may show a ratio greater than one, and if the washing iscarried too far, some sodium may be ion exchanged by hydrogen, and theratio will drop below one. Thus, a typical analysis for a'thoroughlywashed sodium zeolite A is 0.99Na O:1.0Al O :1.85SiO :5.1H O. The ratioThe composition for zeolite A lies in the range of where M represents ametal and n its valence.

Thus the formula for zeolite A may be written as follows:

1.0 i 0.2M O:A1g0a:1.85 :I: 0.5si02;YH2o

In this formula M represents .a metal, n its valence, and Y may be anyvalue up to 6 depending on the identity of the metal and the degree ofdehydration of the crystals.

The pores of zeolite A are normally filled with water and in this case,the above formula represents their chemical analysis. When othermaterials as well as water are in the pores of zeolite A, chemicalanalysis will show a lower value of Y and the presence of otheradsorbates. The presence in the pores of non-volatile materials, such assodium chloride and sodium hydroxide, which are not removable undernormal conditions of activation at temperatures of from 100 C. to 650 C.does not significantly alter the crystal lattice or structure of zeoliteA although it will of necessity alter the chemical composition.

i The apparent density of fully hydrated samples of zeolite A weredetermined by the flotation of the crystals on liquids of appropriatedensities. The technique and liquids used are discussed in an articleentitled Density of Liquid Mixture appearing in Act-aCrystallo-graphica, 1951, vol. 4, page 565. The densities of severalsuch crystals are as follows:

Form of Zeolite A Percent of Density,

Exchange g./cc.

Magnesium 75 2. 045:0. 1 C l i m 93 2. 055:0. 1 Thallous 80 about 3. 36

In making the sodium form of zeolite A, representative reactants aresilica gel, silicic acid or sodium silicate as a source of silica.Alumina may be obtained from activated alumina, gamma alumina, alphaalumina, alumina trihydrate, or sodium aluminate. Sodium hydroxide maysupply the sodium ion and in addition assist in controlling the pH.Preferably the reactants are water soluble. A solution of the reacantsin the proper proportions is placed in a container, suitably of metal orglass. The container is closed to prevent loss of water and thereactants heated for the required time. A convenient and preferredprocedure for preparing the reactant mixture is to make an aqueoussolution containing the sodium aluminate and hydroxide and add this,preferably with agitation, to an aqueoussolution of sodium silicate. Thesystem is stirred until homogeneous or until any gel which forms isbroken into a nearly homogeneous mix. After this mixing, agitation maybe stopped as it is unnecessary to agitate the reacting mass during theformation and crystallization of the zeolite, however, mixing duringformation and crystallization has not been found to be detrimental. Theinitial mixing of ingredients is conveniently done at room temperaturebut this is not essential.

In the synthesis of zeolite A, it has been found that the composition ofthe reacting mixture is critical. The crystallizing temperature and thelength of time the crystallizing temperature is maintained are importantvariables in determining the yield of crystalline material. Under someconditions, for example too low a temperature for too short a time, nocrystalline materials are produced. Extreme conditions may also resultin the production of materials other than zeolite A. I

The sodium form of zeolite A has been produced at C., essentially freefrom contaminating materials, from reacting mixtures whose compositions,expressed as mixtures of the oxides, fall within either of the followingranges.

When zeolite A has been prepared, mixed with other materials, the X-raypattern of the mixture can be repro duced by a simple proportionaladdition of the X-ray patterns of the individual pure components.

Other properties, for instance molecular sieve selee-- tivity,characteristic of zeolite A are present in the properties of the mixtureto the extent that zeolite A is part of the mixture. 7

The adsorbents contemplated herein include not only the sodium form ofzeolite A as synthesized above from a sodium-aluminum-silicate-watersystem with sodium as the exchangeable cation butalso crystallinematerials obtained from such a zeolite by partial or completereplacement of the sodium ion with other cations. The sodium cations canbe replaced at least in part, by other ions. These replacing ions can beclassified in the following groups: other monovalent or divalentcations, such as lithium and magnesium; metal ions in Group I of theperiodic table such as potassium and silver; Group II metal ions such ascalcium and strontium; except barium. The preparation of other cationicmetal zeolites is too complex for their use in the present invention.

The spatial arrangement of the aluminum, silicon, and oxygen atoms whichmake up the basic crystal lattice of the zeolite remains essentiallyunchanged by partial or complete substitution of the sodium ion by othercations. The X-ray patterns of the ion exchanged forms of the zeolite Ashow the same principal lines at essesntially the same positions, butthere are some difi'erences in the relative intensities of the X-raylines, due to the ion exchange.

Ion exchange of the sodium form of zeolite A (which for convenience maybe represented as Na A) or other forms of zeolite A may be accomplishedby conventional ion exchange methods. A preferred continuous method isto pack zeolite A into a series of vertical columns with suitablesupports at the bottom; successively pass through the beds a watersolution of a soluble salt of the cation to be introduced into thezeolite; and change the flow from the first bed to the second bed as thezeolite in the first bed becomes ion exchanged to the desired extent.

To obtain hydrogen exchange, a water solution of an acid such ashydrochloric acid is effective as the exchanging solution. For sodiumexchange, a water solution of sodium chloride is suitable. Otherconvenient reagents are: for potassium exchange, a water solution ofpotassium chloride or dilute potassium hydroxide (pH not over about 12);for lithium, magnesium, calcium, ammonium, nickel, or strontiumexchange, water solutions of the chlorides of these elements; for zincexchange, a water solution of zinc nitrate; and for silver exchange, asilver nitrate solution. While it is more convenient to use watersoluble compounds of the exchange cations, other solutions containingthe desired cations or hydrated cations may be used.

Among the ways of identifying zeolite A and distinguishing it from otherzeolites and other crystalline substances, the X-ray powder difiractionpattern has been found to be -a useful tool. In obtaining the X-raydilfraction powder patterns, standard techniques were employed. Theradiation was the Kot doublet of copper, and a Geiger counterspectrometer with a strip chart pen recorder was used. The peak heightsI and the positions as a function of 26, where 6 is the Bragg angle,were read from the spectrometer chart. From these the relativeintensities, 1001/1 where I is the intensity of the strongest line orpeak, and d (obs), the interplanar spacing in A., corresponding to therecorded lines were calculated.

X-ray powder diffraction data for a sodium zeolite A (Na A), a 95%exchanged potassium zeolite A (K A), a 93% exchanged calcium zeolite A(CaA), a 94% exchanged lithium zeolite A (Li A), a 93% exchangedstrontium zeolite A (SrA), and an exchanged thallium eolite A (TlA) aregiven in Table A. The table lists the IOOI/I and the d values in A. forthe observed line for the different forms of zeolite A. The X-raypatterns indicate a cubic unit cell of a of between 12.0 and 12.4 A. Ina separate column are listed the sum of the squares of the Millerindices (h +k +l for a cubic unit cell that corresponds to the observedlines in the X-ray diffraction patterns. The a values for eachparticular zeolite are also tabulated and in another column theestimated errors in reading the position of an X-ray peak on thespectrometer chart appear.

The relative intensities and the positions of the lines are onlyslightly difierent for these various ion exchanged forms of zeolite A.The patterns show substantially all The spatialappearance of a few minorlines and the disappearance of others from one form of zeolite A toanother as well as slight changes in the intensities and positions ofsome of the X-ray lines can be attributed to the different sizes andnumbers of cations present in the various forms since these differenceseffect some small expansion or contraction of the crystals.

TABLE A Esti- N 82A LizA KzA. mated (h -I-kH-F) il lrrgr 1001/11) dIOOI/I d 1001/1 value 6 4. 26 18 i0. 01 36 4. 02 48 4. 105 33 it). 0043.805 4 3. 895 10 5:0. 003 53 3. 633 53 3. 714 62 i0. 003 3. 555 5 :lzO.003 15 3. 342 28 3. 414 34 i0. 003 47 3. 222 49 3. 292 35 :tO. 002 3.078 12 i0. 002 2. 923 43 2. 985 80 :|:0. 002 2. 837 4 2. 902 27 i0. 00212 2. 691 4 2. 753 :bO. 002 $0. 002

6 1. 653 8 1. 691 7 i0. 001 2 =l=0. 001 4 1. 593 3 1. 631 7 =|=0. 0016 1. 566 3 1.603 6 310.001 4 1. 541 5 1. 576 8 .1. 529 2 1. 492 1 1.481 1. 470 1. 459 3 1. 449 2 1. 438 1.417 g 1. 399 5 1. 403 4 i0. 001 25:0. 001 8 1. 359 7 -l=0. 001

CaA SrA ThA Esti- (h +I: +l mated Error d 1001/1 d 1001/1 d 1001/1 invalue OaA SrA TlzA d 1001/10 d 1001/T (1 1001/1 12. 32 :lzO. 02

(1 Value of Reflection in A.

3.38:0.06 3.26:0.05 2.96:0.05 2.73 tons '2.60i0.05

Zeolite A may be defined as a synthetic crystalline metalaluminum-silicate having an X-ray powder diffraction patterncharacterized by at least those reflections set forth in Table B.

Occasionally, additional lines not belonging to the pattern for zeoliteA, appear in a pattern along with the X-ray lines characteristic ofzeolite A. This is an indication that one or more additional crystallinematerials'are mixed with zeolite A in the sample being tested.Frequently these additional materialscan be identified as initialreactants in the synthesis of the zeolite, or as other crystallinesubstances. 'When zeolite A is heat treated at temperatures of between100 and 600 C. in thepres- 8 ence of water vapor or other gases orvapors, the relative intensities of the lines in the X-ray pattern maybe appreciably changed from those existing in the unactivated zeolite Apatterns. Small changes in line positions may also occur under theseconditions. These changes in no .way hinder the identification of theseX-ray patterns as belonging to zeolite A.

The particular X-ray technique and/or apparatus em ployed, the humidity,the temperature, the orientation of the powder crystals and othervariables, all of which are well known and understood to those skilledin the art of X-ray crystallography or diffraction can cause somevariations in the intensities and positions of the lines. These changes,even in those few instances where they become large, pose no problem tothe skilled X-ray crystallographer in establishing identities. Thus, theX-ray data given herein to identify the A lattice are not to excludethose materials which, due to some variable mentioned or otherwise knownto those skilled in the art, fail to show all of the lines, or show afew extra ones that are permissible in the cubic system of the Azeolite, or show a slight shift in position of the lines, so as to givea slightly larger or smaller lattice parameter.

The zeolites contemplated herein exhibited adsorptive properties thatare unique among known adsorbents. The

A common adsorbents, like charcoal and silica gel, show adsorptionselectivities based primarily on the boiling point or criticaltemperature of the adsorbate, Activated zeolite A on the other handexhibits a selectivity based on the size and shape of the adsorbatemolecule. Among those adsorbate molecules whose size and shape are suchas to permit adsorption by zeolite A, a very strong preference isexhibited toward those that are polar, polarizable, and unsaturated.Another property of zeolite A that contributes to its unique positionamong adsorbents is that of adsorbing large quantities of adsorbateeither at very low pressures, at very low partial pressures, or at verylow concentrations. One or a combination of one or more of these threeadsorption characteristics or others can make to the percentage increasein the Weight of the adsorbent.

The adsorbents were activated by heating them at a reduced pressure toremove adsorbed materials. Throughout the specification the activationtemperature for zeolite peratures, about 196 (3., oxygen but nosubstantial amount of nitrogen is adsorbed. At higher temperatures,about C. or higher, nitrogen is adsorbed in larger quantities thanoxygen. This behavior is demonstrated by the following data:

Pressure Temp. Weight Pressure Temp. Weight Adsorbate (mm. Hg) 0.)Percent (mm. Hg) 0.) Percent Adsorbed Adsorbed A 0 196 24. 8 750 75 4. 8Ng 700 -196 0.6 750 75 10. 6

The preferential adsorption of nitrogen from air at 78 C. was alsodemonstrated in a flow system inwhich air at -78 C. and atmosphericpressure was passed over a bed of sodium zeolite A pellets with asuperficial contact time of 25.6 seconds. The oxygen content of the exitgas rose as high as 89%, and the sorbed gas was as high as 94% nitrogen.With a short contact time of 2 to 7 seconds the first gas emerging fromthe bed was 100% nitrogen as a result of the more rapid rate of oxygenadsorption on freshly activated zeolite at 78 C. This, however, is atemporary condition which changes as the zeolite A approaches itscapacity for oxygen at that temperature.

Potassium zeolite A obtained from other forms of zeolite A by exchangewith a water solution of potassium chloride has a small pore size asshown by the fact that of a large number of adsorbates tested only waterwas adsorbed to any appreciable extent.

The sodium zeolite A, conveniently synthesized from sodium aluminate,sodium silicate and water, has a larger pore size than potassium zeoliteA. The activated sodium zeolite A adsorbs water readily and adsorbs inaddition somewhat larger molecules.

At about room temperature the sodium zeolite A adsorbs the C and Cmembers of the straight chain saturated hydrocarbon series but notappreciable amounts of the higher homologs. Typical results are shownbelow.

In the series of straight chain unsaturated hydrocarbons,

the C and C molecules are adsorbed but the higher homologs are onlyslightly adsorbed. This is shown in the data below for a typical sodiumzeolite A. An exception is butadiene, a doubly unsaturated C TemperaturePressure Weight Percent Adsorbate 0. IIlIH. Hg) adsorbed on Ethylene 25200 p 8. 4 Propylene- 25 200 11.3 Butene-l 25 200 2. 3 Butadiene 25 9.13. 7

In borderline cases where adsorbate molecules are too large to enter thepore system of the zeolite freely, but are not large enough to beexcluded entirely, there is a finite rate of adsorption and the amountadsorbed will vary with time. In general, the recorded data representsthe adsorption occurring within the first one or two hours, and for someborderline molecules, further adsorption may be expected during periodsof ten to fifteen hours. Washing techniques, diiferent heat treatmentsand the crystal size of thesodium zeolite A powder can cause veryappreciable differencesin adsorption rates for the borderline molecules.

The calcium and magnesium exchanged zeolite A'have molecular sieveadsorptive properties characteristic of materials with largerpores thanexist in sodium'zeolite A. These two forms of divalent ion exchangedz'eoliteA behave quite similarly and adsorb all molecules adsorbed bysodium zeolite A plus some larger molecules.' For instance, in additionto adsorbing oxygen at liquid air temperature, nitrogen and krypton arealso adsorbed. Typical data for an 85% exchanged calcium zeolite A,

' of the Zeolites.

prepared fnom sodium zeolite A with a solution of calcium chloride aregiven below.

This data. obtained on a 66% Ca exchanged Zeolite A.

The calcium and magnesium forms of zeolite A have a pore size that willpermit adsorption of molecules for which the maximum dimension of theminimum projected cross-section is approximately 4.9A. but not largerthan about 5.5A. The approximate maximum dimension of the minimumprojected cross-section for several molecules is as follows:benzene-5.5, propane4.9, ethane4.0, and iso-butane-5 .6.

There are numerous other ion exchanged forms of zeo' lite A such aslithium, ammonium, silver, zinc, nickel, hydrogen and strontium. Ingeneral, the divdent ion exchanged materials such as zinc, nickel andstrontium zeolite A having a sieving action similar to that of calciumand magnesium zeolite A, and the monovalent ion exchanged materials suchas lithium and hydrogen zeo lite A behave similarly to sodium zeolite A,although some differences exist.

The molecular sieving characteristics of zeolite A may be influenced bythe temperature and pressure at which the adsorbent is activated asshown by oxygen adsorption data for sodium zeolite A.

Activation Weight Percent 02 Temperature Adsorbed on N 82A at (Pressure0.01 mm. Hg). 196 C. and 13 mm.

C. Hg Pressure The sample of sodium zeolite A activated at the lowertemperature does not adsorb oxygen while the sample activated at thehigher temperature does. This is true even though both samples adsorbover 24% by weight of water at 25 C. and 24 mm. of Hg water vaporpressure.

Another unique property of zeolite A is its strong preference for polar,polarizable and unsaturated molecules providing of course that thesemolecules areof a size and shape permitting them to enter the poresystem This is in contrast to charcoal and silica gel which show a mainpreference based on the volatility of the adsorbate. The following tablecompares the adsorptions of water, a polar molecule on charcoal, silicagel and sodium zeolite A. The table illustrates the high capacity thezeolite A has for polar molecules.

A selectivity for polar, polarizable arid unsaturated molecules is notnew among adsorbents. Silica gel exhibits some preference for suchmolecules, but the extent of this selectivity is so much greater withzeolite A that 0 separation prowsses based upon this selectivity becomefeasible.

The selectivity for polar, polarizable and unsaturated molecules can bealtered appreciably by ion exchange and in addition relativeselectivities may change with temperature.

Zeolite A shows a selectivity for adrsorbates, provided that they aresmall enough to enter the porous network of the zeolites, based on theboiling points of the adsorbates as well as on their polarity,polarizability or degree of unsaturation. For instance, hydrogen whichhas a low boiling point is not strongly'adsorbed at room temperature. I

A further important characteristic of zeolite A is its property ofadsorbing large amounts of adsorbates at low adsorbate pressures,partial pressures or concentrations. This property makes zeolite Auniquely useful in the more complete removal of adsorbable impuritiesfrom gas and liquid mixtures. It gives them a relatively high adsorptioncapacity even when the material being adsorbed from a mixture is presentin very low concentrations, and permits the efiicient recovery of minorcomponents of mixtures. This characteristic is all the more importantsince adsorption processes are most frequently used when the desiredcomponent is present in low concentrations or low partial pressures.High adsorptions at low pressures or concentrations on zeolite A areillustrated in the following table, along with some comparative data forsilica gel and charcoal.

molecule, air, nitrogen and hydrogen. As previously discussed andillustrated by the tables, zeolite A is capable of adsorbing all ofthese compounds, based on a consideration of the zeolite A pore size andcritical molecular dimensions of these compounds. That is, the pores ofzeolite A are sufficiently large and in fact do receive methane, ethane,propane, ethylene, propylene, air, nitrogen or hydrogen molecules.Furthermore, it is known that zeolite A is strongly selective forolefinic hydrocarbons such as ethylene and propylene.

Based on these considerations, one skilled in the art would notrecognize that zeolite A would possess its par- 1 Pu=the vapor pressureof water at the temperature given.

The strong adsorption of water by zeolite A at low pressures can becapitalized on to remove water from mixtures with other materials.

The adsorption capacity of adsorbents usually decreases with increasingtemperature, and while the adsorption capacity of an adsorbent at agiven temperature may be suilicient, the capacity may be whollyunsatisfactory at a higher temperature. With zeolite A a relatively highcapacity may be retained at higher temperatures. For instance,adsorption data for water on calcium zeolite A and silica gel at C. and100 C. are tabulated below. It is seen that the capacity of calciumzeolite A remains high even at 100 C.

1 P zthe vapor pressure of water at 25 C.

Zeolite A may be activated by heating it in either air, a vacuum, orother appropriate gas to temperatures of as high as 600 C. Theconditions used for desorption of an adsorbate from zeolite A vary withthe adsorbate, but either raising the temperature and reducing thepressure, partial pressure or concentration of the adsorbate in contactwith the adsorbent or a combination of these steps is usually employed.Another method is to displace the adsorbate by adsorption of anothermore strongly held adsorbate. a

Zeolite A may be used as an adsorbent for the purpose indicated above inany suitable form. For example, a

ticular selectivity for water in preference to the other constituents ofthe present vapor mixture. Contrary to these expectations, it has beendiscovered that zeolite A possesses an extremely strong selectivity forwater to the A substantial exclusion of olefinic and normal saturatedaliphatic hydrocarbons having less than four carbon atoms per molecule,air, nitrogen and hydrogen.

Another interrelated property of zeolite A which is utilized by thepresent invention is the relationship of the boiling point or vaportension characteristics of an individual fluid or clearly related typeof fluid to the capacity of the crystalline zeolite A to adsorb thefluid at a given temperature and pressure. More specifically, it hasbeen discovered that a relationship exists between the amount of fluidadsorbed and the temperature ratio T /T where T is the temperature indegrees Kelvin during adsorption, assuming that the temperature of thefluid and the adsorbent are in equilibrium. T is the temperature indegrees Kelvin at which the vapor pressure of the fluid is equal to thepartial pressure or vapor tension of the fluid in equilibrium with thezeolite A adsorbent. Stated in another way, T is the temperature atwhich the vapor pressure of the adsorbate is equal to the partialpressure of the adsorbate during adsorption. T is actually the dew pointof the adsorbate at the adsorption conditions.

This relationship is clearly shown in FIG. '1 which is a plot of theweight percent of water adsorbed versus the temperature ratio T T 1 forboth monovalent'and divalent cationic forms of zeolite A. The followingTable C is a summary of the data from which FIG. 1 was prepared, thedata having been assembled from tests described in more detail in otherparts of the specification. That is, the T; values for these exampleswere obtained from the preceding portion of the specification and the Tvalues were read from the vapor pressure tables in Industrial andEngineering Chemistry, vol. 39, page 517, April 1947.

TABLE C Temp., K. Ion Form of Water Ten- Wt. Percent Tr/Ti Zeolite Asion in mm. Water Hg Adsorbed T1 T2 An inspection of Table C will revealthat it covers T values from 25 C. to 350 C. and adsorbate pressuresfrom 0.01 mm. Hg to 23.7 mm. Hg. It was unexpectedly discovered thatwater being freely adsorbed exhibits the same temperature ratio T Trelationship to weight percent adsorbed for all four cationic forms ofzeolite A. The same relationship should exist for the remaining cationicforms of zeolite A, since each has very similar adsorptivecharacteristics to at least one of the tested forms. The presentinvention utilizes this relationship in combination with the previouslydiscussed water selec- Q tivity property of zeolite A to provide a novelprocess for separating water from a vapor mixture containing at leastone member of the group consisting of olefinic and normal saturatedaliphatic hydrocarbons containing less than four carbon atoms permolecule, air, nitrogen and hydrogen. In its broadest form, the processconsists of contacting the vapor mixture with a bed of at leastpartially dehydrated crystalline zeolite A adsorbent material. Thewater-depleted vapor mixture is then discharged from the bed. Suchcontact is preferably eifected under conditions such that thetemperature ratio T T with respect to the inlet end of the bed and withrespect to the Water contained in the vapor mixture is between 0.50 and1.0, where T is the adsorption temperature and is less than 873 K.

and T is the temperature in degrees Kelvin at which water has a vaporpressure equal to its partial pressure in the vapor mixture. The lowerlimit of 0.50 for the temperature ratio T /T is fixed by the discoverythat below this value there is a smaller'percentage change in adsorptioncapacity per unit change in the temperature ratio. Conversely above0.50, there is a larger percentage change in adsorption capacity perunit change in the temperature ratio. Stated in another way, if it isdesired to obtain a certain incremental adsorbate loading at a specifiedadsorption temperature with a given feed stream, it would be necessaryto increase the pressure of operation by a greater percent if thetemperature ratio is below 0.50 than if it is maintained above thisvalue in accordance with the invention. Also, the temperature ratio of0.50 corresponds to a bed loading of 3.2 weight percent adsorbate, andif the temperature ratio were reduced below this value, a largeradsorption bed would be required with its attendant higher investmentand operating expenses.

The upper limit of 1.0 for the temperature ratio should not be exceeded,because if the adsorption temperature is equal to or less than the dewpoint, Water condensation will occur, thereby essentially eliminatingthe sieving action of the zeolite A adsorbent. The broad upper limit of873 K. for T is due to the fact that above this temperature, the crystalstructure of zeolite A will be disrupted or damaged with consequent lossof adsorption capacity and 14 616 K. but higher than 233 K. This is forthe reason that above such range, the zeolite A crystal structure willbe damaged by virtue of the contact with water, thereby resulting inpermanent loss of capacity and increase in mass transfer resistance.Below 233 K., relatively economical refrigerants such as Freon-12 cannotbe employed, thereby necessitating more expensive refrigeration systems.Also, the mechanical properties of metals deteriorates rapidly belowabout 233 K., so that special reduction in pore size, therebyfundamentally changing its adsorptive characteristics.

The present adsorption process is most efiiciently performed it T theadsorption temperature, is less than construction materials must beemployed for adsorbers operating in this low temperature range. However,the increase in zeolite A adsorptive capacity for Water at reducedtemperatures justifies the employment of refrigeration down to the 233K. level.

The present invention also contemplates a process for continuouslyseparating water from a vapor mixture containing at least one memberselected from the group consisting of olefinic and normal saturatedaliphatic hydrocarbons containing less than four carbon atoms permolecule, air, nitrogen and hydrogen. This continuous process includestwo steps, an adsorption stroke and a regeneration stroke. Theadsorption stroke is the same as the previously described adsorptionwhere the temperature ratio T /T is between 0.50 and 1.0, and T is lessthan 873 K. In the regeneration stroke, at least part of the adsorbedwater is removed by subjecting the zeolite A adsorbent to conditionssuch that the temperature ratio T T at the end of the regenerationstroke, with respect to the adsorbed Water, is less than the temperatureratio at the end of the adsorption stroke. Also, for the broad range thedifierence in total adsorbate loading between the ends of the adsorptionand regeneration strokes is at least 0.02 Weight percent for increasedefliciency of the overall process. A lower differential adsorbateloading would entail prohibitively large adsorber units. During theregeneration stroke T is the regeneration temperature and is less than873 K., and T is the temperature at which the adsorbed water has a vaporpressure equal to the partial pressure of the water over the zeolite Abed at the end of the regeneration. It will be understood by thoseskilled in the art that at least two adsorbent bed-s may be provided,with one bed on adsorption stroke and the other bed on regenerationstroke. The respective flows are then periodically switched when thefirst bed becomes loaded with the adsorbate, so that the latter isplaced on regeneration stroke and the second bed is placed on-stream.

The continuous process is most efiiciently performed if T the adsorptiontemperature is less than 616 K. but higher than 233 K., for previouslystated reasons. Also, for maximum efiiciency of the'overall process, thedifierence in adsorbate loading between the ends of the adsorption andregeneration strokes is at least 0.5 weight percent. During theregeneration stroke, T is preferably below 616 K. for the previouslydiscussed reasons, and above 283 C. It has been discovered that therequired duration of the regeneration stroke is substantially longer ifthe regeneration is performed below ambient conditions.

It will be understood by those skilled in the art that the temperatureratio may be adjusted by well-known methods, as for example, heating thebed by direct or indirect heat transfer, employing a purge gas, or bydrawing a vacuum on the bed during the regeneration stroke. Also, duringthe regeneration stroke the ratio may be adjusted for favorableoperation by varying either or both the temperature and the pressure.

The many advantages of the invention are illustrated by the followingexamples.

EXAMPLE 1 A methane feed stream is provided at 25 C. and 1 atmospherepressure, and with a water dew point of 0 C. The stream is to be driedby contact with a zeolite A bed ata temperature of 25 C. The adsorbentis to be regenerated by employing the 0 C. dew point feed stream.

Using these given conditions, the potential capacity of the bed toadsorb water at the bed inlet section may be determined as follows: TheT /T value during the adsorption stroke is 273/298 or 0.92. Thistemperature ratio provides a loading of 26.8 Weight percent water on thezeolite A adsorbent as determined by a reading of the FIG. 1 graph. Thepotential capacity of the adsorbent bed inlet end for methane may bedetermined in a'similar manner by using the previously reference vaporpressure table so that T is 112 K. and the potential methane capacity isonly about 0.8 weight percent. If the desired dew point is 50 C., theadsorption stroke may be terminated when the dew point of the efiluentgas rises to this value.

Since the zeolite A adsorbent has an extremely high capacity formoisture, it is not necessary that the bed be completely regenerated.For this reason, the adsorbent bed need only be regenerated to aresidual moisture loading of, for example, about 3.2 weight percent. Forthis value, T /T is equal to 0.50 and since T is 273 K., the Tregeneration temperature must be 546 K. or 273 C.

EXAMPLE II An ethylene feed stream is provided at 25 C. and 1 atmospherepressure and with a water dew point of C.

The stream is to be dried by contact with a zeolite A bed at atemperature of 25 C. The adsorbent is to be regenerated by employing the0 C. dew point feed stream.

Using these given conditions, the potential capacity of the bed toadsorb water at the bed inlet section may be determined as follows: TheT /T value during the adsorption stroke is 273/298 or 0.92. Thistemperature ratio provides a loading of 26.8 weight percent water on thezeolite A adsorbent as determined by a reading of the FIG. 1 graph. Thepotential capacity of the adsorbent bed inlet end for ethylene may bedetermined in a similar manner by using the previously referenced vaporpressure table, so that T is 169 K. and the potential ethylene capacityis only about 9 weight percent. If the desired dew point is --40 C., theadsorption stroke may be terminated when the dew point of the efiluentgas rises to this value.

For the same reasons as described in conjunction with Example I, theadsorbent bed need only be regenerated to a residual moisture loadingof, for example, about 3.5 weight percent. For this value, T T is equalto 0.52 and since T is 273 K., the T regeneration temperature must -be525 K. or 252 C.

If the inlet vapor mixture were to contain nitrogen, the potentialcapacity of the zeolite A adsorbent for this constituent could besimilarly determined by reference to the vapor pressure tables and FIG.4. Also, the potential capacity for air and hydrogen may be obtained inan analogous manner.

Although the preferred embodiments have been described in detail, it iscontemplated that modifications of the process may be made and that somefeatures may be employed without others, all within the spirit and scopeof the invention as set forth herein.

This is a continuation-in-part application of copending applicationSerial No. 400,385, filed December 24, 1953, in the name of R. M.Milton, now abandoned.

What is claimed is:

1. A process for separating water from a vapor mixture containing waterand at least one olefinic hydrocarbon having less than four carbon atomsper molecule, which comprises contacting said vapor mixture with a bedof at least partially dehydrated crystalline zeolite A adsorbentmaterial having pores sufficiently large to receive the olefin, andthereafter discharging the water-depleted vapor stream from said bed.

2. A process for separating water from a vapor mixture containing waterand ethylene, which comprises contacting said vapor mixturewith a bed ofat least partially dehydrated crystalline zeolite A adsorbent materialhaving pores sufficiently large to receive ethylene, and thereafterdischarging the water-depleted vapor stream from said bed.

3. A process for separating water from a vapor mixture containing waterand propylene, which comprises contacting said vapor mixture with a bedof at least partially dehydrated crystalline zeolite A adsorbentmaterial having pores sufficiently large to receive propylene, andthereafter discharging the water-depleted vapor stream from said bed.

References Cited in the file of this patent UNITED STATES PATENTS1,813,174 Lamb July 7, 1931 2,988,503 Milton et al June 13,1961

3,024,867 Milton Mar. 13, 1962 3,024,868 Milton Mar. 13, 1962 FOREIGNPATENTS 555,482 Canada Apr. 1, 1958 OTHER REFERENCES The HydrothermalChemistry of Silicates, Part I, by Barrer et al., Journal of theChemical Society, 1951, pp. 1267-1278.

Separation of Mixtures Using Zeolites as Molecular Sieves, Part 1, ThreeClasses of Molecular Sieve Zeolite; J. Soc. Chem. Ind., vol. 64, May1945, pages 130, 131.

Examine These Ways to Use Selective Adsorption, Petroleum Refiner, vol.36, No. 7, July 1957, pages 136- 140.

1. A PROCESS FOR SEPARATING WATER FROM A VAPOR MIXTURE CONTAINING WATERAND AT LEAST ONE OLEFINIC HYDROCARBON HAVING LESS THAN FOUR CARBON ATOMSPER MOLECULE, WHICH COMPRISES CONTACTING SAID VAPOR MIXTURE WITH A BEDOF AT LEAST PARTIALLY DEHYDRATED CRYSTALLINE ZEOLITE A ADSORBENTMATERIAL HAVING PORES SUFFICIENTLY LARGE TO RECEIVE